This invention, in general, pertains to the fabrication of superconducting magnets utilizing thin film conductors. In particular, we describe and demonstrate a technique that allows for the continuous winding of undulator magnets using HTS Rare-Earth (RE) Barium Copper Oxide (REBCO) coated conductor tapes.
The availability of powerful, partly coherent x-ray and deep UV beams has enabled tremendous advances in science and technology across a broad range of disciplines, including materials science, biology, pharmacology and nanoscience. The central components in storage rings and/or free electron laser based light sources that actually generate high-brilliance x-ray radiation are the undulators. Undulators establish a spatially periodic pattern of magnetic fields along the trajectory of relativistic electrons, which induces a periodic component in the electron motion that in turn results in the emission of electromagnetic radiation. An important figure of merit describing an undulator is the deflection parameter K given as K=0.934λu [cm] B0 [T], where λu is the period of the magnetic pattern and B0 is the peak on-axis magnetic field. The wavelength range over which the emitted radiation is tunable and the brightness of the emitted beam depend on K; therefore, the on-axis field of the undulator. Therefore, the design and fabrication of undulators delivering high on-axis magnetic fields is a central task in the planning of next-generation light sources. Undulators comprised of superconducting solenoids that are wound onto ferromagnetic cores (for short superconducting undulator, SCU) have emerged as the most promising approach for reaching high B0, and, at the same time, high uniformity (low phase errors) of the device. Currently, a large majority of the developmental work on SCUs utilizes NbTi superconducting wires. The wire technology of NbTi is highly developed; however, this material, which has comparatively low superconducting transition temperature of Tc˜10 K and upper critical field of 14 T, reaches its material limitations particularly in regards to the high-field critical current density at an operating temperature of 4.2 K, and further performance enhancements of SCU will require the use of different superconducting materials. Nb3Sn with a Tc of 18 K and an upper critical field in excess of 25 T is being employed for high-field magnets and for large-scale magnets in high-energy physics experiments and fusion reactor technology. Efforts at LBNL and at Ohio State University pursue Nb3Sn for SCUs. Even though Nb3Sn is a well-established material allowing for significantly enhanced performance as compared to NbTi, its metallurgy is complicated. For instance, magnets have to be fabricated in the wind-and-react approach in which a precursor wire is wound into the final shape that is subsequently reacted into the Nb3Sn phase in a high-temperature (650-700° C.) annealing step. Dimensional deformations of the undulator core that can occur during this annealing step interfere with the need for very tight mechanical tolerances of the undulator, an issue that has not been resolved yet and also after high-temperature annealing step, wire becomes brittle which results in performance degradation.
So-called high-temperature superconducting materials offer an alternative to the traditional Nb-based superconductors. In regards to wire development, the most promising candidates are MgB2 (Tc˜35-39 K), either round wire or tape, Bi-2212 round wire (Tc˜80-90 K) or REBCO coated conductor tapes (Tc˜90 K). In addition to improved undulator performance in terms of high-field critical current density, that is, achievable on-axis field, an important benefit arises from the high-transition temperature, which enables operation at a temperature higher than 4.2 K, thereby making possible the use of cryogen-free cryocoolers, and enabling the use of a less complex and cheaper cooling system than is currently required for NbTi-magnets. Furthermore, operation at a temperature higher than 4.2 K eliminates cooling problems related to the heat load generated by the electron beam, which could reach as high as 45 W in the case of a beam injection accident. Managing this head-load requires expensive cryogenic equipment for operating NbTi-undulators. The higher stability margin of HTS will make future HTS-undulators less sensitive to electron-beam-induced heat loads. For example, at an operation temperature of 10 K, the undulator could be in direct contact with the electron-beam pipe, without the need of intervening vacuum spaces.
The current performance of MgB2 wires does not surpass that of NbTi yet. Bi-2212 round wires require a wind-and-react process in which the superconducting material is formed in a high-temperature annealing step (˜900° C.,). Thus, this wire incurs the same challenges as Nb3Sn (see above). REBCO coated conductors offer an attractive opportunity to realize next generation SCUs. This conductor does not require an annealing process and it is ready to wind as received. Coated conductors are composed of a highly engineered layered structure aimed at achieving as perfect as possible a textured structure of the REBCO layer. A possible drawback is that in the final conductor only roughly 1% of the cross-section is superconducting. Nevertheless, tremendous improvements have been made in the current-carrying capability of REBCO conductors by modifying their microstructure such that the engineering critical current density, Je, that is, the critical current density per wire, exceeds that of NbTi under the anticipated operating conditions.
Thus, REBCO coated conductors can enable the next phase in undulator technology. A major hurdle in realizing this potential arises from the difficulties in transferring well-established magnet technologies developed for Nb-based wires to the tape-shaped REBCO-conductors. In particular, due to their large aspect ratio (width versus thickness) the tapes tolerate only small side-bending (bending within the plane of the tape); furthermore, normal bending is typically limited to bend diameters larger than 0.5″ in order to avoid irreversible degradation of the critical current. Therefore, the traditional layer-by-layer winding approach is, in most cases, not feasible with coated conductors, and HTS magnets are typically wound as a stack of so-called pancake coils. In each pancake coil the tape is wound on itself from the inside out to the desired outer diameter of the magnet. Connecting the pancake coils to each other and to the current leads is challenging, and various schemes have been proposed. If all the winding stacks have the same polarity, one can—at the expense of some side-bending—wind two winding stacks that are connected on the inner most winding layer through an inclined section of conductor by using two feed bobbins resulting in a double winding stack coil. In one design, successive double winding stacks are connected through a soldered bridge joint on the outer surface of the magnet. In a planar undulator successive pancake coils have opposite polarity (winding direction) in order to generate the periodic magnetic pattern, and winding stacks need to be connected through soldered bridge joints on the inner winding layer and on the outside. Such a scheme has recently been realized in a collaboration between the ANKA synchrotron in Karlsruhe, Germany, and Babcock Noell GmbH, Wurzburg, Germany. However, this design did not perform well as compared to NbTi-based undulators since the achieved Je was only about 700 A/mm2.
A major drawback that is likely to prevent the scalability of this design to a full-scale undulator is the large number of soldered bridge joints, two per pancake coil. In contrast to Nb-based superconductors, it is currently not possible to make truly superconducting joints between sections of coated conductors in the environments typical for coil winding; the soldered bridge joints are resistive. A recently reported procedure for making superconducting joints requires delicate post-processing involving high-temperature post-annealing in oxygen and under pressure. Detailed procedures for splicing coated conductors have been established and contact resistivities as low as 40-50 nΩcm2 can be achieved. For a 4-mm wide conductor and 5 cm splice length this would imply a resistance of 50 nΩ per bridge joint, or a power dissipation of 0.1 W per pancake coil at an operating current of 1000 A. This level of dissipation exceeds the cooling capacity of the cryocoolers currently used for NbTi-undulators, and in fact, maybe off-set the anticipated higher temperature margins of HTS undulators. On general grounds, the resistive joints may be regarded as weak spots in the undulator. The localized heat generation may cause quenches of the superconductor nearby. Furthermore, the mechanical stiffness of the joint is very different than that of the isolated coated conductor, which could cause damage due to differential thermal contraction on cool-down.
Here, we disclose and demonstrate a new technology for continuous coil winding of REBCO coated conductor tapes for the fabrication of SCUs. This technology overcomes the problems related to winding of these tape-shaped superconductors and that have prevented to fully-utilize the superior materials properties of REBCO for the next generation of SCUs.